The prior method and devices do not allow to measure CO.sub.2 continuously during heating cycles: pyrolysis or oxidation. Furthermore, the CO that is released simultaneously with CO.sub.2 cannot be measured. This analysis of the CO that is produced only by cracking organic matter is notably essential for an improved determination of the total organic carbon. This analysis allows, for example, to obtain a more precise oxygen index OI by including in the CO.sub.2 the CO thus analyzed Simple interpretation in combination of the continuous CO and CO.sub.2 curves allows the organic oxygen to be differentiated from the inorganic oxygen.